1. Field of the Invention
The present invention relates to a process for the preparation of water-proof sheets. More particularly, the present invention relates to a process for the preparation of water-proof sheets in which a fibrous substrate fabric constituting the water-proof sheet has no water-absorbing property.
2. Description of the Related Art
Water-proof sheets have heretofore been prepared by forming a coating of a high polymer on the surface of a fibrous substrate fabric. However, in conventional water-proof sheets having coatings formed on both the surfaces of a fibrous substrate fabric, if the coatings are ruptured or broken even slightly, water is absorbed into the fibrous substrate fabric as the intermediate layer from this rupture or breakage, or even if this rupture or breakage is not formed, water is absorbed from the cut section of the sheet. If the fiber density is high, this absorption of water is enhanced by the capillary phenomenon. Absorption of water results not only in increase of the weight of the water-proof sheet but also in reduction of the adhesion between the fibrous substrate fabric and the coating. Furthermore, absorbed water is hardly discharged from the fibrous substrate fabric and propagation of mildews is promoted, and dirty water or colored water is absorbed to degrade the appearance of the sheet.
As means for eliminating this defect, there has been adopted a method in which a fibrous substrate fabric is treated with a water repellant and a polymer coating is formed on both the surfaces. In this case, since the fibrous substrate fabric is treated with a water repellant, permeation of water is prevented, but if the water repellent effect is increased, the adhesion between the coating and the fibrous substrate fabric is reduced and also the bonding force of the coating is reduced, and the product is not preferred from the practical viewpoint. Furthermore, there is adopted a method in which an adhesive substance such as an isocyanate is incorporated into the water repellant or coating material to improve the adhesion, but even according to this method, no satisfactory results can be obtained.